Plasma treatment is well known as a surface treatment method, such as cleaning and etching, on an object to be processed, for example, a substrate on which electronic components are mounted. In plasma treatment, a substrate as an object to be processed is set in a vacuum chamber that forms a treatment room, and plasma discharge is generated in the treatment room. Ions and electrons generated by plasma discharge allow the object to have a predetermined surface treatment. For plasma treatment with high quality and stability, plasma discharge has to be generated suitable for discharge conditions determined in advance so as to fulfill a processing purpose.
To check for a generating state of plasma discharge, various methods have been employed. For example, followings are well known: a method for finding an influence on voltage and current of a high-frequency power supply section due to changes in plasma discharge from any cause, and a method for judging a discharge condition by detecting self bias voltage generated between electrodes by the plasma discharge.
According to the methods above, however, when plasma discharge has to be generated under low-output conditions, accurate detection of the discharge condition cannot be expected due to poor detection accuracy. In that case, a method capable of directly detecting changes in condition of plasma discharge is employed. Specifically, a discharge detecting sensor is disposed in the vacuum chamber of the processing room. The discharge detecting sensor has a probe electrode for detecting change in electrical potential caused with change in plasma discharge, thereby detecting an abnormal discharge in the processing room. Through the method, change in condition of plasma discharge in the processing room can be detected with high sensitivity. Therefore, in principle, even when the high-frequency power supply section supplies low output, the presence or absence of plasma discharge and abnormal discharge is properly monitored. The method is introduced in patent literature 1.
However, patent literature 1 has no specific description on the followings: a specific example of application required for monitoring with high accuracy of the presence or absence of plasma discharge and discharge abnormalities; a problem of having no normal discharge at the start of the device; and measures against an abnormal discharge occurred during operations.    Patent literature 1: Japanese Patent Application Publication No. 2003-318115